Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS).
FIG. 1 illustrates schematically a UMTS network 1 that comprises a core network 2 and a UMTS Terrestrial Radio Access Network (UTRAN) 3. The UTRAN 3 comprises a number of Radio Network Controllers (RNCs) 4, each of which is coupled to a set of neighbouring Node Bs 5. A Node B is effectively a Base Transceiver Station. Each Node B 5 is responsible for a given geographical cell and the controlling RNC 4 is responsible for routing user and signalling data between the Node B 5 and the core network 2. A User Equipment (UE) 6 communicates via a Node B 5 using a radio link, and it is desirable to improve speeds of communication over the radio link between the UE and the Node B.
Since the 1999 release of the basic 3GPP specifications for WCDMA, there have been several releases which improve on various aspects of that 1999 release. In release 5 of the WCDMA 3GPP specifications, high speed downlink packet access (HSPDA) was introduced to reduce downlink delays and increase downlink data rate capability by approximately a factor of three. Release 6 of the WCDMA 3GPP specifications also reduces uplink delays and increases uplink data rate capability by approximately a factor of two.
Release 6 introduced a new uplink transport channel called the Enhanced Dedicated Channel (E-DCH) targeted for interactive, background, and streaming traffic. Compared to the normal uplink DCH, the E-DCH achieves improved uplink performance using a short transmission time interval (TTI), hybrid ARQ with soft combining, and scheduling. Reducing the TTI allows for an overall reduction in delay and faster hybrid ARQ retransmissions. Fast hybrid ARQ with soft combining reduces the number of retransmissions as well as the time between retransmissions. It also allows for a significant increase in capacity. Fast scheduling allows for rapid resource reallocation between UEs 6, exploiting the “burst” properties of packet data transmissions. It also allows the system to admit a larger number of high data rate users and adapt rapidly to interference variations, thereby leading to an increase in capacity as well as an increase in the likelihood that a user will experience high data rates. The functionality for controlling retransmission delay for hybrid ARQ and fast scheduling is implemented in the Node B 5.
In the downlink HSPDA, the transmission power and the code space is the shared resource, but in the uplink E-DCH, the interference “headroom” is the amount of shared resource (i.e., transmit power or interference) left to be allocated to one or more mobile terminals to transmit in the uplink. This is realized in the form of spreading codes. Even though the spreading codes may be completely orthogonal in theory, a user always interferes with another user to some extent in reality. This is due to the fact that time shifts of the codes are not perfectly orthogonal. Furthermore, owing to time dispersion inherent in the radio channel, replicas of the time shifted signal will in most cases be received at the receiver side.
In a WCDMA uplink, each UE 6 has its own scrambling code. The scrambling codes consist of long sequences of pseudo-random chips. Cross-correlation properties of the scrambling codes ensure that any two scrambling codes are almost orthogonal, no matter the time shift. However, as mentioned above, they are never fully orthogonal. Consequently, a UE 6 transmitting data in the uplink will always interfere with all other users to some extent. This will ultimately set a limit on the number of UEs that can be supported in a cell, assuming a shared interference headroom. The amount of interference that a user generates as seen by another user is hence determined by the degree of non-orthogonality of the time shifted replicas of the scrambling codes, but also the power transmitted on the scrambling code.
In order to support many users, it is important to keep the interference as low as possible. As the orthogonality properties of the scrambling codes cannot be changed within the framework of the current 3GPP standard, it is necessary to keep the transmitted power as low as possible.
The common uplink resource shared among the UEs 6 is the total amount of tolerable interference, i.e., the total received power at the Node B 5. The amount of common uplink resources allocated to a UE 6 depends on the data rate (transport format) to be used. Generally, the higher the data rate, the larger the required transmission power/interference, and thus, the higher the resource consumption.
Scheduling is the mechanism that determines when a certain UE 6 is allowed to transmit, and at what maximum data rate. Packet data applications are typically bursty in nature with large and rapid variations in their resource requirements. The goal of the uplink scheduler is therefore to allocate a large fraction of the shared resource to users momentarily requiring high data rates, while at the same time ensuring stable system operation by avoiding sudden interference peaks. Identifying this goal is one thing; achieving it is another.
The uplink dedicated channels DCHs in WCDMA are “fast” power-controlled, meaning that the base station measures the received DPCCH signal quality, e.g., the received signal to interference ratio (SIR), and compares the measurement to a desired signal quality, e.g., a SIR target value. If the measured SIR is less than or equal to the SIR target, the Node B 5 signals an “up” power control command to the UE 6 to make the UE 6 increase the power by a predefined step, or a “down” power control command to the UE 6 to make it decrease its power by a predefined step if the received SIR is greater than the SIR target. The SIR target is regularly updated in a “slow” power control procedure known as outer loop power control (OLPC).
The WCDMA uplink typically comprises of several physical channels, examples of which include a Dedicated Physical Control Channel (DPCCH), a Dedicated Physical Data Channel (DPDCH), a High Speed Dedicated Physical Control Channel (HS-DPCCH), an Enhanced Dedicated Physical Control Channel (E-DPCCH) and an Enhanced Dedicated Physical Data Channel (E-DPDCH). It is not necessary for all channels of these channels to be present for a specific connection, but there is always one DPCCH present on each radio link. For a “pure” Enhanced uplink (EUL) Radio Access Bearer (RAB), there may be, for example, a DPCCH, an E-DPCCH and one or more E-DPDCH(s). The DPCCH comprises 10 bits per slot which are used as pilots for transmit power control (TPC) command, transport format combination indicator (TFCI) and feedback information (FBI), where the latter two are not always present. The pilot bits are used, among other things, for channel estimation and for determination of the SIR for the sake of power control of the uplink.
If there are few EUL UEs in the cell transmitting at a high data rate, these EUL UEs allocate most of the power to the E-DPDCH(s), and most of the interference experienced by other users comes from the power allocated to the E-DPDCH(s) of other users (neglecting own-interference such as ISI). However, in some cases the UEs transmit at a low or moderate rate. This may occur, for example, when the UE is using an email client. The client may send small signals once very 60 seconds or so to check for new emails. In this case, the power fraction that a UE allocates to the data channel (E-DPDCH(s)) decreases, and the power fraction that a user allocates to the control channels increases. Consequently, an increased fraction of the interference in the cell is a result of power transmitted on the control channels of other UEs. If there are many UEs in the cell, the interference from the control channels may dominate the total interference. In an extreme case, enough UEs may use an increased DPCCH power fraction that the interference headroom is consumed by the DPCCH of the active UEs. In that case, the scheduler is not able to give any UEs a grant to transmit user data on the E-DPDCH.
One way to reduce the interference of the DPCCH channel is the introduction of Continuous Packet Connectivity (CPC), as described in Release 7 of the 3GPP standard. This allows a UE to refrain from transmitting on the DPCCH when there is no user data to be transmitted on the E-DPDCH. However, in order to maintain synchronization, DPCCH bursts are still required on a regular basis. This causes interference. Furthermore, in the TTIs when user data is transmitted on the E-DPDCH, there is no benefit to using CPC, as transmissions are required on the DPCCH.